Method of and apparatus for testing magnetic objects.



APPLICATION FILED JAN-7| I915- Patented Aug. 29, 19116.

4 SHEETS-SHEET I- ATTORNEY F. P. FAHY.

METHOD 0F AND APPARATUS FOR TESTINGMAGNETLC OBJECTS.

APPLICATION FILED JAN. 7. 1915.

o Patented Aug. 29, 1916.

4 SHEETS-SHEEf 2.

nAJMAAMAM-A [Jhllnhah ATTORNEY F. P. FAHY.

METHOD OF AND APPARATUS FOR TESTING MAGNETIC OBJECTS.

' APPLICATION Fl-LED JAN. 1. 1915.

Patented Aug. 29

4 SHEETS-SHEET 3.

' WITNESSES ATTORNEY F. P. FAHY. METHOD OF AND APPARATUS FOR TESTINGMAGNETIC OBJECTS.

I APPLICATION FILED JAN. 7, l9l5- I Patented Aug. 29, 1916.

' 4 SHEETS-SHEET 4.

I I I m I I V V I 11F INVENTOR A ATTORNEY I METHOD OF AND APPARATUS FORTESTING MAGNETIC OBJECTS.

Specification of Letters Patent.

Patented na so, rare.

Continuation in part of application Serial No. 721,823, filed September23, 1912. This application file January 7, 1915.

To all whom it may concern Be" it known that T, FRANK P. FAHY, a citizenof the United States of America, residing in the city of -Washington, inthe District of Columbia, have invented certain new and usefulImprovements in Methods of and Apparatus for Testing Magnetic Ob: jects,of which the following is a true and exact description, reference beinghad to the accompanying drawings, which form a part thereof. I

The primary object of my invention is to provide a practical andefiective method of and apparatus for measuring, or obtaining anindication of the magnetic characteristics of metallic bodies, with aview to determining certain magnetic properties of the metallic bodiesor certain mechanical properties of these bodies which depend upon orvary with their magnetic characteristics.

More specifically, the object of my invention is to provide acomparatively simple and reliable device suitable for comparativelyrapid manipulation and reading, and adapted for use as an instrument ofprecision both for comparing the magnetic characteristics of two bodies,and as an absolute permeameter to determine the permeability of a singlebody.

In carrying out my invention T provide a stationary core of magneticmaterial which may be laminated. if the conditions of use make thisdesirable, but for most purposes need not be laminated. This core isshaped to provide two similar magnetic circuits each including anelongated individual air gap and a core portion common to both circuits. Tn the use of this instrument as a magnetic comparator, one ofthese air gaps is bridged by one, and the other air gap is bridged bythe second of the magnetic bodies to be compared. Associated with thecore of the apparatus are provisions for subjecting each branch circuitto the same magnetomotive force, and for inductively com paring themagnetic effects produced thereby in the two branch magnetic circuits.

The means employed in practice for subjecting the two branches of themagnetic circuit to similar magnetizing forces and for obtaining ameasure of the flux variations produced therebyin the two magnetic circuits, comprise a magnetizing windingv'connected to asuitable extraneoussource of electromotive force, and an induced current Serial No. 894.

winding having its terminals connected to a suitable galvanometer orother indicating instrument. One or the other of these two w ndings mustinclude two portions, one applled in inductive relation to one, and theother applied in inductive relation to the second of the separatebranches of the magnetic circuit. The second of the two windings mayinclude portions partly applied to one and partly applied to the otherof the two branches of the magnetic core, or may be wholly applied tothe common portion of the core Tnstead of employing the apparatus re-.

air gaps of the apparatus will furnish an indication 'of the differencebetween their magnetic characteristics. When the apparatus has once beencalibrated, so to speak, by the use of readings obtained with a standardpiece, or in some other manner, the readings obtained when the air gapof one only of the branch magnetic circuit is bridged will of themselvesfurnish a definite indication of the magnetic characteristics of.

the'object bridging the air gap and if a suitable compensation is madefor the tendency to dissimilar magnetic leakages in. the two branchcircuits, the apparatus may be used as an instrument of precision indetermining the absolute permeability of said object.

My invention comprises efiective provisions for compensating for thetendency to dissimilar magnetic leakages in the 'two branch magneticcircuits when one only of the two end air gaps is bridged by magneticmaterial, or when there is a considerable difi'erence in thepermeability of the two bridges across these-air gaps.

' The simultaneous use of standard and test pieces having about the samemagnet characteristics possesses the important" practical advantageofincreasing the sensitiveness of the apparatus, since the quantity thenrel l mfi b measured by the galvanometer.

"or other indicating instrument may be the integrated electromotiveforce due to the difierential of the flux variations in the two separatemagnetic circuits. Where the standard and test pieces difi'er slightlyin cross section, this variation in cross section may be compensated forby correspondingly .varying the number of turns in different piecebridges one air gap and the other air gap is left unbridged byimpressing a known integrated counter electromotive force on the inducedcurrent circuit to thereby minimize thequantity directly measured by thegalvanometer or other indicating instrument employed.

The various features of novelty which characterize my invention arepointed out with particularity in the claims annexed to and forming apart of this specification.

For a better understanding of the invention, however, and its specificobjects and j the advantages possessed by it, reference should be had tothe acompanying drawings and descriptive matter in which I haveillustrated and described various embodiments of the invention.

Of the drawings: Figure 1 is a plan view partly in section of apreferred form of apparatus embodying my invention. Fig. 2 is anelevation of the apparatus shown in Fig. 1. Fig. 3 is. a diagrammaticrepresentation of the apparatus shown in Fig. 1, with certain preferredexternal electric circuit connections thereto. Fig. 4 is a diagrammaticrepresentation of the apparatus shown in Figs. 1 and 2 modified by theaddition of certain windings. Fig. 4 is a view taken similarly to Fig. 4showing a portion of a modified arrangement. Fig. 5 is a perspectiveview of a polar portion of the magnetic core at one end of one of thecore arms C, illustrating a different pole face construction from thatshown in Figs. 1 and 2. Fig. 6 is a a view taken similarly to Fig. 5 andillustrating still another pole face construction. Fig. 7 is a view of aportion of the modified form of core construction especially devised foruse in determining the magnetic characteristics of helical springs. Fig.8 is a view of a portion of the magnetic'core of an instrument as shownin Figs. 1 and 2 illustrating the use of non-magnetic spacers to produceair gaps in the branch magnetic circuits. Fig. 9 is a sectionalelevation of a portion of the apparatus shown in Fig. 1, the sectionbeing taken on the line 9-9 of Fig. 1, and Figs. 10, 11, 12 and 13 arediagrammatic representations each showing the application to the core ofsuch an instrument as is shown in Fig. 1 of a difi'erent which is of Hshape, is formed at its ends with threaded portions 13' screwed intothreaded sockets formed in the transverse core portions, each of whichcomprises one arm C forming a portion of one, and a second an arm Cforming a portion of the other of the two magnetic circuits of which theyoke B forms a common ortion. As shown, the core parts B and areunlaminated.

For reasons explained hereinafter it is practicallv essential for some,and desirable for most conditions of use to provide an air gap in thecore,'and for many conditions it is desirable tb adjust this air gap inorder to vary its, reluctance. For this purpose the core B is dividedmidway between its ends into two sections separated by the small air gapB and one of the sections is formed with a cavity B in its inner end.This cavity receives a piston like body I) of magnetic material which isadjusted toward and away from the adjacent end surface of the othersection of the core portion B in order to form a more or less effectivemagnetic shunt about the air gap 13. The shunt piece 6 is formed with astem portion 1) passing through an axial passage B in the correspondingpart of the core portion B. The outer end of the stem 6 is provided witha handle 12 by means of which the stem may be rotated and therebyaxially adjusted through the engagement of the threaded inner portion bof the stem 6 with the thread:

ed inner end of the passage.B*. The adja cent ends of the two sectionsof the core portion B are formed with" registering sockets receivingnon-magnetic dowel-plus B holding the core parts in the desired relativeposition.

As shown, each core piece C C is mounted on a specially shapedsupporting member or beam (1 of non-magnetic material secured to theplatform or base A, being secured thereto by means of bracket members aand a of non-magnetic material which are fastened by machine screws bothto the supporting member a and to the core portion Q G. The

brackets a secured to the base A adjacent each end of each core piece CC are provided with threaded sockets for clamping screws D by means ofwhich standard and test pieces may be clamped against the ends of eitheror both, the pairs of core arms C C and C C, or against pole shoe partssecured thereto as hereinafter explained. In Figs. 1 and 2 a test pieceX is clamped against the en s of the core arms 0, and a icense standardpiece S is clamped against the ends of the core arm C.

The cross bar or yoke portion B of the core of the instrument, as shownin Figs. 1 and 2, is surrounded by a magnetizing coil E, and one of thecore arms C is surrounded by a test coil F and the alined core arm C issurrounded by a similar test coil- F. Four similar compensating coils' Iare mounted one on each of the four core arms. The test and compensatingcoils are each secured in place by corresponding pairsof split collars Jclamped about the core arms, the ends of the usualspool of each of thesecoils being engaged by a corresponding pair of clamping rings J. Thetest piece X is surrounded by a coil G. The end flanges g of the spoolof the coil G- are advantageously secured to the ends of the core arms(I as by means of screws 9. When the coil Gr is made to fit about a testpiece of oblong shape in crosssection, as shown and as is frequently thecase, the flange 9 may well be provided with a curved slot g to permitthe coil. to be secured in place with the long dimension of the crosssection of the coil passage either transverse, or parallel to the lengthof the core arms C as shown by the full and dotted line positionsrespectively of Fig. 9.

H represents a winding exactly similar in the number and area of itsturns to the winding G surrounding the standard piece X and having theend flanges h of its spool secured to the core arms C by screws 71. asthe spool flanges g of the winding G are secured to the core arm 0.Preferably as shown, the spool it also supports a measuring coil HAcomprising more turns than the coil H and having its area turns care- Ifully determined.

The ends of the diflerent windings are connected," as shown, to suitablebinding posts mounted on the base A, (see Figs. 1

and

KE represents the binding posts to which the magnetizing winding E isconnected. The four compensating coils I are connected in series betweenthe binding posts KI. The

two test coils F and F are connected in series between the binding postsKF and KF". One terminal of the coil G is connected to the binding postKF, and the other terminal to the binding post KG. One terminal ofthelcoil H is'connected to the binding post KF and the other to thebinding post KH. The terminals of the coil HA are connected to thebinding posts KHA, It is to be observed. also that all of the testwindings, namely: the coils F, F G and Hare con nected in series betweenthe binding posts KG and KH, and in such manner that on a reversal ofthe magnetizing current through coil E, the integrated electromotiveforces induced in the coils F and H ,actin the same direction and inopposition to the integrated electro-motive forces simultaneouslyinduced in the coils F and G.

In Fig. 3 I have illustrated an advantageous arrangement of externalinstrument and circuit connections to the binding posts of the magnetictesting instrument proper. As shown in this figure, M represents avariable resistance, 0 an ammeter, P a source or electro-motive current,and L a reversing switch, all so connected to the binding posts KE thatthe resistance M, ammeter O and source of current P, may be connected inseries with the magnetizing winding E with the current flow in eitherdirection through the winding E from the source P. L represents areversing switch and N an adjustable resistance so connected to thebinding posts KI, and to the switch L that the compensating coils I andthe variable resistance N, while in series with the ammeter O andsourceof current P, are in shunt to the winding E, and by manipulatingthe switch L the direction of current flow through the compensatingcoils T may be reversed without a corresponding reversal of the currentflow through the winding E. The binding posts KG, KF, KF, KH and KHA areconnected to'a suitable galvanometer or other current indicating instru'ment Q through the switches U, L and L in such a manner, as will bereadily apparent to those skilled in the art, that the instrument Q maybe connected at will either to the terminals of the coils F and F, tothe terminals of the coil G, to the terminals of the coil H, to theterminals of-the coil HA, orto the terminals of the circuit includingthe coils F, F, G and H all connected in series.

In the use of the apparatus illustrated for the magnetic comparison ofobjects one bridging the air gap between the arms C, and the other thegap between arms Ggthe object, generally speaking, is toestablish thesame difference in magnetic potential between the portions of the objectcontacting with or facing the ends of the core arms C,

as between the similar portions of the object bridging the air gapbetween the core arms G, and then to measure the magnetic fluxes throughthe two objects. Similarly in the use of the apparatus for obtainingabsolute measurements, the operation is to establish between theportions of an object contacting with, or facing the ends of the corearms C, the same difl'erence in magnetic potential as exists betweenthefree ends of the core arms C',and then to measure the magnetic fluxpassing through the said object, and the magnetic flux passing through acertain definite and restricted portion of the air path between the corearms C. The'impression of the magnetizing force on the common portion Bof the two magnetic circuits'will its .netic circuits which will bemanifested by different leakage characteristics in such circuits. Themagnetic characteristics of each of these paths will depend ofcours'e,upon whether or not the corresponding core arms are connected bya standard or test specimen, and if so, upon the permeability, size andshape of the specimen, and the character of the contact between thespecimen and the ends of the corresponding core arms.

The use of the apparatus as an instrument of precision for comparingstandard. and test pieces of different magnetic characteristics, or forobtaining an absolute measure ment of the magnetic, properties of a testpiece, therefore, requires a compensation for dissimilar magneto-motiveforce drops in similar parts of the two magnetic circuits. Thiscompensation can be very effectively secured, as hereinafter explained,by the use of the compensating coils I. I' will first describe modes ofusing the apparatus, however, without bringing the compensating coils Iinto service, i. e., with the switch L left open.

In comparing the magnetic properties of two objects as for instance, thestandard and test pieces S and X shown in Figs. 1 and 2, I first clampthem snugly in place as shown, and it may be explained at this pointthat the pressure with which the bars are clamped in place is in generalimmaterial, all that is required being to insure firm contact and toprevent accidental displacement.

I then adjust the circuit conditions as by manipulating the variableresistance M to obtain the current in the magnetizing coil E giving thedesired magneto-motive force when the switch L is closed. I thenmanipulate the switch L to reverse the direction of current flow throughthe winding E several times so as .to bring the standard-and test piecesinto a cyclic state magnetically.

-A reading is then taken of the deflection of the measuring instrument Qwhen connected to the binding posts KF and KF and hence in series withthe test coils F and F or readings are taken when the instrument, isconnected to the terminals of the 0011 G and to the terminals of thecoil H, or preferably all three .of these'readings are taken,

the magnetizing current through the winding E being reversed once inmaking each reading. Either the one read ng when the instrument isconnected to coils F and F instead of which are connected to buck oneanother, or the difference between the two readings taken with theinstrument connected first to the one and then to the other of the coilsG and II will fairly indicate the difference in the magneticcharacteristics of the standard and test pieces when the magneticleakage of the two magnetic circuits are not very dissimilar. When allthree readings are taken, however, the first furnishes a check on the,resultant of the second and third readings, and a comparison of thedifferent readings indicates the character of the leakage conditionsprevailing. When the current through the coil E is reversed theinstrument Q will, if of a suitable character, indicate the resultant ofthe integrated electro-motive'forces generated in the coils connected inseries therewith on the flux changes therethrough resulting from thereversal of the magnetizing current. The integrated value of theresultant electro-motive forces generated in the coils F and F connectedin series will be proportional to the difference between theelectro-motive forces generated in the coils G and H disregardingunequal leakage conditions.

The most satisfactory indicating device Q which I have found forpractical purposes, is a ballistic galvanometer having a relatively longperiod, but it will be understood that other formsof electricalmeasuring instruments may be employed, and in particular, that analternating current instrument must be employed when the coil E isconnected to a source of alternating current. With instrument Q asuitable ballistic alvanometer, the direction and extent o the needledeflection will be dependent upon the resultant flux changes in theportion or portions of the magnetic circuits surrounded by test coilsconnected to the instrument.

It is desirable for accurate work, to keep the magnetizing .current flowthrough the coil E fairly constant. This involves the use of-the ammeterO and adjustment of the resistance M in the particular apparatus shown.However, since a variation in the strength of the magnetizin currentwill under certain conditions of ux affect both the standard and testpieces to a practically equal extent, it is one of the advantages of myinvention that a close adjustment of the magnetizing current isunnecessary. Instead -of employing the battery P as indi cated in Fig.3, a commercial lighting or power circuit may be employed as a source ofthe magnetizing current. even when the voltage of such a circuitfluctuates considerably.

When it is desired to obtain a knowledge of the magnetic characteristicsof an ob ect, as the test piece X, by direct measurement y comparisonwith a standard iaeaaae piece in'the manner described, the standardpiece is removedand the magnetizing force to which the piece Xis-subjected and the resulting induction therein may then be indicatedby the readings of the galvanometer when the coils HA and G arerespectively connected thereto, and the current through the magnetizingcoil E reversed in each case.

As already stated, however, the use of the apparatus as an instrument ofprecision, either as a magnetic comparator or as an absolutepermeameter, requires that a compensation be made for unequalmagnetomotive force drops which may exist in corresponding parts of thetwo branch magnetic circuits and which result in dissimilar leakagecharacteristics of the two magnetic circuits. The compensation of thiskind, necessary to a high degree of precision, can be readily obtainedby means of the compensating coils I. These coils are preferably eachmounted close to the free end of the core arm C or C surrounded by it,and are connected so that when a current flow through them takes place,they tend to create a magnetic flux passing in one direc-.

tion or the other along the closed path indicated by the arrows appliedto Fig. 4. In use, the reversing switch L is closed in such a fashionthat the magnetizing effect of these compensating coils augments themagnetizing efliect of the winding E on one, and subtracts from itsmagnetizing effect on the other of they two branch magnetic circuits.Which of the two circuits has its magnetizing force so strengtheneddepends upon the extent and distribution of the magnetomotive forcedrops in different portions of said branch circuits.

After the switch L is closed to send current through the compensatingcoils in the proper direction, the current flow through the compensatingcoils is adjusted, as by manipulation of the resistance N, until the'galvanometer Q, when the latter is connected in series withv the coils,F, F, G and H indicates that the integrated value of the resultantelectro-motive forces in said coils is zero when switch L is manipulatedto reverse the current flow through the main magnetizing coil E andthrough the compensating coils I. electro-motive forces generated in theop positely connected coils F and G is a function of the leakage inshunt to the test piece X, and the resultant of the electro-motiveforces inducedin the oppositely connected coils F and H is a similarfunction of the leakage in shunt to the standard pieces. When theintegrated value of these opposing resultant electro-motive forcesbecomes zero, this means that the leakage characteristics of the twobranch magnetic circuits are equivalent, and sinceunder these conditionsthe two leakage paths are the similar The resultant of the air paths onebetween the two poles formed by the ends of the core arms 0 and theother,

dition is brought about, the reading obtained more, the galvanometer Q,when connected to the terminals of the coil G will directly indicate theinduction through the test piece X. Similarly a comparison of thereadings obtained by the galva'nometer when, con nected to the coil HAand tb-the coil G' will give the relation between the magnetizing forceand the induction produced thereby in a body X surrounded by the'coil G,the body S being removed, in the use of the instrument as an absolutepermeameter to determine the permeability of the body X.

While there should be substantial identity in shape between standard andtest pieces for the most exact use of the instrument as a magneticcomparator, this is to be understood as necessarily applying only to theportions of the pieces between and in proximity to the pole piece. Itwill be apparent that the shape, and indeed the composition of theportions, if any, of the standard and 7 test pieces S and Xrespectivelyextending away from the outer sides or edges of the core arms C and C,in any setting of the apparatus, will affect the readings obtained onlyby virtue of the magnetic leakagefrom these portions. the resultsobtained will take place at suchpro ecting portions only of the standardends or test pieces as are in immediate proximity to the core legs, andif comparatively short portions of the projecting ends ofthe standardpiece adjacent the core leg C are similar to the corresponding portionsof the Leakage sufiicient to affect test piece X, differences in theouter. portions of the projecting ends of the standard and test piecesbecome unimportant. i

The efiect of the end extensions of objects such as long bars or railsbeing measured or compared, may be avoided or substantially minimized bythe use of the supplementary similar coils- R and R shown in Fig. 4:.The coils R and R are coaxial with the coils G, and H respectively, andwhen in use are connected in series with the coils F, F, G and H, thecoil B being so wound and connected that the electro-motive force generated in it by the leakage flux is added to that generated in the coil Gand opposes that generated in the coil F, while the coil R is wound andconnected to aid the coil der test or comparison and lying beyond thecore leg C is substantially avoided, for the magnetic leakage throughwhat may be. regarded as the normal air path. for leakage between theends of the arms C is the same as the magnetic leakage through thesimilar leakage path. between the ends of the arms G, and the equalityof leakage means an equality in magnetic potential difference. Either orboth coils R and R may be cut out when the corresponding bodies undertest or comparison are of such form as shown in Figs. 1 and 2 and do notproject by the ends of the core arms connected by them, or if left incircuit, the coils then act like so much simple resistance.

It is customary in determining the magnetic characteristics of objectsby tests made under an absolute testing method, to make them under fixeddefinite temperature conditions. In using my instrument, however, incomparing standard and test pieces, the temperature effect tends tobecome unimportant, as both the standard and test pieces are afiected topractically an equal extent by changes in temperature above or belowthat used ordinarily in making tests by absolute methods.

While the arrangement of magnetizing and test coils illustrated in Figs.1 and 2 and diagrammatically in Fig. 3 is that Which I prefer to employin the practical use of my invention, it will be obvious to thoseskilled in the art, that the magnetizing and testing windings may be.arranged in many different ways and in Figs. 10 to 13, I haveillustrated difierentwinding arrangements which I may employ. Thewinding arrangement shown in Fig. 10 differs from that shown in Figs. 1and 2, primarily in the fact that the two test coils F and F aresupplemented by similar test coils F and F one on eachof the core arms Cand C. These coils are so connected that the electro-motive forcegenerated incoil F is augmented by that induced in the coil F which ison the other arm C while the coils F and F one on each arm C aid oneanother in opposing coils F and F Compensating coils such as coils I ofFigs. 1 and 2 can be effectively employed with the test coils F, F, Fand F of Fig. 10 provided test coils corresponding to the coils G and Hare also employed.

In the modified winding arrangement illustrated in Fig. 11 themagnetizing coil E of Fig. 1 is replaced by four similar magnetizingcoils E, E E and E connected in series and located one upon each of thefour core arms C and C. In this figure, there is employed a single testcoil FA symmetrically disposed upon the yoke B and connected to theballistic galvanometer or other measuring device Q. The magnetizingcoils E, E E and E are connected in such a manner that they develop inthe magnetic circuit a flux which is closed upon itself as indicated bythe arrows.

When the flux change in the pieces S and X is the same, upon a change inthe magnetizing force of the coils E, E E and E", no flux passes throughthe common core portion B since the magnetic potential differencebetween the ends of the cross bar B is zero. hen the flux change in thepieces S and X differs, however, a flux will pass through the coreportion B since ,there will be no longer a zero potential differencebetween the ends of the core portion B and consequently the needle ofthe indicating device Q Wlll be deflected. The direction of needledeflection and its extent will be dependent, of course, upon thedifference in the flux changes in the two branches of the magneticcircuit.

Another example of an arrangement of the magnetizing and induced currentwindings is shown in Fig. 12 wherein the coils E, E E and E, arearranged as in Fig. 11, and are surrounded respectively by testing coilsF, F, F and F arranged as in Fig. 10.

lVith one air gap only between the ends of the arms bridged by amagnetic body, the quantity measured by the instrument Q,

. is much larger, and the sensitiveness of the primary of the variablemutual inductance meaaaa 4 equal to the integrated electro-motive forcewhich would be induced in a test cdilhaving a number of turns similar innumber of turns to the coils F and F if such a coil were uniformly woundon a standard bar S when arranged as shown in Figs. 1 and 2. The coil Fand the secondary of the mutual inductance T are so connected in seriesthat the sum of their integrated electro-motive forces opposes theelectro-motive force gen-' erated at the same time in the coils F.

Considering the device purely as a magnetic comparator the use of avariable inductance in place of a standard piece, possesses a number ofadvantages: First, the

necessity for a standard specimen of the known composition and size isavoided and the test specimens are compared to a fixed standard of fluxchange. Second, the mutual inductance being variable, it may becalibrated and adjusted so that its setting in any particular test willtake into account the area of the test specimen. When the device is usedas an absolute permeameter compensation such as is afiorded by the coilsI of Figs. 1, 2 and 3 is required and this necessitates the use ofadditional coils G and H or equivalent test coils.

In'testing or comparing long bars, it may be desirable to avoid illeffects from dissimilar contacts with the two core arms bridged by eachbar'to put a test coil on each core arm as in Fig. 10, and to divide thecoils R and R applying to the two halves of each divided coil, one toone, and the other to the second of the projecting ends of the bar. Thisdivision of the coils is illustrated in Fig. 4? wherein F and Frepresent the test coils on the core arms C, and r and 1" represent thetwoparts into which the coil R of Fig. a has been divided.

The instrument disclosed herein is capable of use for many diiferentpurposes. lit may be employed to determine the ma etic characteristicsof materials intende for use under conditions where their magneticproperties are the important characteristics. For instance, it may beused to determine or compare the permeability, residual induction, andcoercive force of magnetic materials. Where results of the highestaccuracy are required in the use of the in strument as a magneticcomparator, the standard piece may be of known-magnetic characteristicsdetermined by the most accurate absolute method known.

The apparatus is well adapted and was in part devised for use indetermining such physical properties as elasticity, ductility andtenacity, and that somewhat indefinite property called. hardness, in sofar as these properties are varied in a metallic body of given basicchemicall composition b the heat treatments to which it has been subjected by comparing this object with another object which is of the sameshape and the same basic chemical composition but has been subjected toknown or standard heat treatment. For instance, the objects compared maybe similar sections of the same rolled'bar and the standard bar annealedwhile the test piece is hardened.

This application is to be regarded in part as a continuation of myearlier application, Serial No. 721,823, filed September 23, 1912,wherein I claim the method just described, of determining changes in thephysical characteristics of objects of the same basic chemicalcomposition resulting from the heat treatments to which they have beensubjected;

The apparatus may be used also to determine the existence, and to someextent the .to roughly indicate the chemical composition of ferruginoussubstances containing elements aflecting their magnetic characteristicssuch as carbon, manganese, silicon, nlckel, tungsten, vanadium andchromium.

To accommodate the difierent uses to which a single instrument may beput, I prefer to provide for the use of special pole faces of magneticmaterial to suit the form and characteristics of the pieces tested. ThisI- do in the apparatus shown in Figs. 1 and 2 by forming open endedgrooves or slots extending parallel to the length of the standard andtest pieces which may be connected thereto. When the pieces to be testedor compared are bars rectangular in cross section as shown in Figs. 1and 2, I fill these grooves or slots with smooth fitting plugs C seeFig. 8; of magnetic materials which extend flush with the end and edgesurfaces of the core arm C.

In testing and comparing round bars, I replace each plug C by such aplug C formed with rounded grooves to receive and fit the rod to betested, as is'shown in Fig. 5.

In Fig. 6, I show a U shaped piece C of magnetic material adapted to bereceived in the core arm cavity C and especially intended for use inholding the end of a group or bundle of laminae Z when it is desired toexamine the magnetic characteristics of such lamina. It is found inmagnetizing a bundle of laminae that efi'icient metal to metal contactcan only be obtained through the edge surfaces of the laminze. When thelatter are assembled as shown in Fig. 6, one end edge of each lamina isin contact with L the corresponding core arm at the bottom of the recessC therein, while the other end edge of each lamina is in direct contactwith the yoke portion of the piece-C Where the instrument is especiallyintended for use in testing or comparing magnetic properties of helicalsprings as to determine the temper thereof, I may provide the threadedpolar extensions C as shown in Fig. 7, each of these extensionsterminating in a head formed with a recess or seat receiving acorresponding end of the spring W.

By providing the magnetic core of the apparatus with a definite gap orgaps symmetrically disposed with respect to the branch circuits such asis formed by the air gap B in the core yoke B, a number of importantadvantages are obtained. One of these arises from the fact that sincethe major portion of the reluctance of the magnetic circuit as a whole,when both test and standardpi'eces are on, is then found in such a gapor gaps the tendency of the gap or gaps is to maintain a practicallyconstant magneto-motive force drop between the ends of the common coreportion, and any ordi-- nary variation in reluctance in successive testspecimens of the same general character will not modify themagneto-motive force drop except in a very small degree. This makes itpossible to subject the objects tested in successive. tests to apractically constant magnetic potential difference, without altering themagnetizing current, which is obviously conducive to uniform results.

A second advantage, due to the maintenance of a practically uniformmagnetic potential diiference in successive tests, is had from the factthat the magnetic leakage conditions in the difi'erent tests remainpractically constant. i

A third advantage had with the gap or gaps in the magnetic core, is dueto the fact that'a pole is formed at each gap end 'at the instant atwhich the magnetizing circuit is broken. These poles act in a mannertending to demagnetize both the core of the apparatus and the test andstandard pieces, thereby lowering the time constant of the magneticcircuit and permitting the use'of an indicator of shorter period thanwould otherwise be required.

A fourth advantage of the provisionof a gap or gaps in the magnetic coreis due to the fact that the gaps minimize the effect of poor contact ofeither the test or standard piece, or both, by reason of scale or ruston such pieces, since the added reluctance due to such'poor contact isbut a small part of the total reluctance present. 1

A fifth advantage of the provision of the gap or gaps in the magneticcore is due to the fact that the gaps render the magnetic quality of thematerial entering into the construction of the core of minor impormessesmits the residual magnetic effect remaining when the magnetizing circuitis broken to be reduced to zero and thereby facilitates a determinationof the true residual induction of the test specimen. The adjustabilityof the gap permits the demagnetizing effect of the gap to be varied atwill which is of especial utility in hysteresis determinations.Preferably I employ a single gaplocated centrally in the cross bar A, asshown in Figs. 1 and 3, since this is the most advantageous position tocontrol. leakage and to provide a higher effective magneto-motive forcenear the pole shoes than at the center of the test or standard bar, soas to compensate for the reluctance due to necessarily imperfect contactat such shoes and to make the flux in the test and standard pieces moreuniform. I may also employ with the apparatus shown in Fig. 1, gaps dueto the insertion of brass or other non-magnetic spacers Y, see Figs. 4and 8, between the test or test and standard pieces and thecorresponding polar ends of arms C and C. Where four similar spacers Yare inserted one between each core arm C and C and the test and standardpieces being compared, as shown in Fig. 4 these spacers similarlyincrease the reluctance of the two magnetic circuits and may be used toaugment the effect of,-or in lieu of the gap B The gap s be tween a testor standard piece and the corresponding core arms have the specialadvantage that they also tend to a more uniform flux distribution fromone end to the other of the tested standard bodies.

While in accordance with the provisions of the statutes I have disclosedand explained the best forms of my invention now known to me, it will beapparent to those skilled in the art that changes may be made in themodes of operation and forms of 'appamaaaaa cuits each including a coreportion common to both circuits and an air gap individual to eachcircuit, one of which is adapted to be bridged by an object to betested, means for impressing substantially similar magnetizing forces onthe two separate magnetic circuits and means for measuring thedifferential of the inductive effects of the flux variations in saidcircuits.

2. Means for determining magnetic characteristics of a metallic objectcomprising in combination a stationary magnetic core' shaped to providetwo separate magnetic circuits each including a core portion common toboth circuits and an air gap individual to each circuit, one of which isadapted to be bridged by an object to be tested, a magnetizing windingfor impressing similar magnetizing forces on the two separate magneticcircuits, and a winding in which an electro-motive force is created byflux variations in said core, one of said windings including one portionin inductive relation with one of said separate magnetic circuits and asecond portion in inductive relation with the other of said separatemagnetic circuits.

3. Means for determinin magnetic characteristics of a metallic objectincluding in combination a stationary magnetic core comprising a corebody and a pair of oppositely disposed branches extending from each endthereof, whereby two separate magnetic circuits are formed each of whichincludes one of said branches at each end of said core body and the airgap between their outer ends, said core body being common to bothcircuits, a magnetizing winding for impressing substantially equalmagnetizing force on the two branch circuits, and an induced currentwinding adapted to have an electromotive force induced thereinproportional to the diflerential of the flux variations in said magneticcircuits, an indicating instrument connected to the last mentionedwinding, and means for minimizing the electromotive force impressed onsaid instrument when one of said air gaps is bridged by the object to betested by a definite amount to approximately compensate for the effectof the change in reluctivity in said magnetic circuit occurring whensaid object bridges the air ga thereof.

4. l/leans for determining magnetic characteristics of a metallic objectcomprising in combination a stationary magnetic core shaped to providetwo separate magnetic circuits each including a core portion common toboth circuits and an air gap individual to each circuit, one of which isadapted to be bridged by an object to be tested, spacers of highreluctivity adapted to be interposed between the said object and thecore at the ends of the said air gap bridged by the object, means forsubjecting the two magnetic circuits to the same magnetizing forces, andmeans for measuring the differential of the inductive effects of theflux variations in said circuits.

5. Means for comparing the magnetic characteristics of two objectscomprising in combination a magnetic core shaped to provide two similarmagnetic circuits each including a core portion common to both circuitsand an air gap individual to each circuit, said air gaps being adaptedto be bridged one by one and the other by the second of the objects tobe compared, spacers of high reluctivity interposed between said objectsand said core to similarly increase the reluctivityof the separatemagnetic circuits, means for impressing substantially similarmagnetizing forces on the two separate magnetic circuits, and means formeasuring the differential of the inductive effects of the fluxvariations in said circuits.

6. Means for determining magnetic characteristics of a metallic objectincluding in combination a stationary magnetic core structure comprisinga body portion and an opposed pair of lateral branches extending fromeach end thereof whereby two separate includes one of said lateralbranches at each end of said core body and the air gap between theirouter ends, said core being common to both circuits and said corestructure including a portion or portions of relatively high reluctivityand similarly increasing the reluctivity of the two separatemagneticcircuits, means for impressing substantially similar magnetizingforces on the two separate magnetic circuits, and means for measuringthe differential of the inductive effects of the fiux variations in saidcircuits.

7. Means for comparing the magnetic properties of two objects comprisingin combination a stationary magnetic core shaped .to provide two similarmagnetic circuits each including a core portion common to both circuitsand an air gap individual to each circuit and adapted to be bridged byone of the objects tobe compared, means for impressing substantiallysimilar magnetizing forces in the two magnetic circuits, and means forcomparing the inductive effects of flux variations in the two circuits.

8. Means for determining magnetic characteristics of a metallic objectcomprising in combination a magnetic core shaped to provide two similarmagnetic circuits each including a core portion common to both circuitsand an air gap individual to each. circuit one of which is adapted to bebridged by the object to be tested, means for impressing substantiallysimilar magnetizing forces on the two separate magnetic circuits, andmeans for comparing the inductive efi'ects of flux variations in the twocircuits, said core being formed with a gap or gaps similarly increasingthe reluctance of both of said magnetic circuits.

9. Means for determining magnetic characteristics of a metallic objectcomprising in combination a magnetic core shaped to. provide two similarmagnetic circuits each including a core portion com 1110112 to bothcircuits and an air gap individual to each circuit one of which isadapted to be bridged by the object to be tested, means for impressingsubstantially similar magnetizing it'orces on the two separate magneticcircuits, and means for comparing the inductive effects of fluxvariations in the, two circuits, said core comprising relativelyadjustable parts adapted to efleet similar variations in the reluctanceof both of said magnetic circuits.

10. Means for determining magnetic properties of two objects comprisingin combination a magnetic core shaped to provide two similar magneticcircuits each including a core portion common to both circuits and anair gap individual to each circuit one of which is adapted to be bridgedby'an object to be tested, means for impressing substantially similarmagnetizing forces on the two branch circuits, and means for comparingthe inductive effects of flux variations in the two circuits, saidcommon core portion being formed with a gap similarly increasing thereluctance of both branch circuits.

11. Means for comparing the magnetic properties of two objectscomprising in combination a magnetic core shaped to provide two similarmagnetic circuits each including a core portion common to both circuitsand an air gap individual to each circuit and adapted to be bridged byone otthe objects to be compared, means for impressing substantiallysimilar magnetizing forces on the two branch circuits. and means forcomparing the inductive efi'ects of flux variations in the two circuits,said common core portions comprising two similar portions of magneticmaterial placed end to end and spaced a determined distance apart.

12. Means for comparing the magnetic properties of two objectscomprising in combination a magnetic core shaped to provide two similarmagnetic circuits each including a core portion common to both circuitsand an air gap individual to each circuit and adapted to be bridged byone of the objects to be compared, means for impressing substantiallysimilar magnetizing forces on the two branch circuits, and means forcomparing the inductive effects of flux variations in the two circuits,said common core portions comprising two portions of magnetic materialplaced end to end and spaced :1 determined distance apart, and anadjustable magnetic body forming a magnetic shunt about the gap betweensaid portions.

13. Means for comparing the magnetic properties of two objectscomprising in combination a magnetic core shaped to provide two similarmagnetic circuits each including a core portion common to both circuitsand an air gap individual to each circuit and adapted to be bridged byone of the objects to be compared, means for impressing substantiallysimilar magnetizing forces on the two branch circuits, and means forcomparing the inductive efiects of flux variations in the two circuits,said common core portions comprising two portions of magnetic materialplaced end to end and spaced a determined distance apart, one of saidportions being formed with a longitudinal passage therein and a magneticbody adjustably received in 'said passage and forming a magnetic shuntabout the gap between said portions.

14. Means for determining magnetic properties of an object comprising incombination a magnetic core shaped to provide two similar magneticcircuits, each including a core portion common to both circuits and anair gap individual to each circuit one of which is adapted to be bridgedby an object to be tested, a magnetizing coil surrounding said commoncore portion, and other coils surrounding separated portions of the twocircuits and so relatively connected that the'voltage induced therein bythe flux variation in each of said magnetic circuits opposes the voltageinduced by the flux variation in the other circuit and means formeasuring the current flow induced in said other coils by inequalitiesin the flux variations in the two circuits.

15. Means for determining the magnetic characteristics of a metallicobject comprising in combination a stationary magnetic core, shaped toprovide two separate magnetic circuits each including a core portioncommon to both circuits and an air gap individual to each circuit, meansfor impressing substantially similar magnetizing forces on the twoseparate magnetic circuits tending to magnctize the common core portionin the same direction, means for impressing on said magnetic circuitsmagnetizing forces tending in one. circuit to aid the correspondingfirst mentioned force, and in the other circuit to oppose thecorresponding first mentioned force, and means for comparing theinductive effects of the flux variations in said circuits. I

16. The method of determining the mag netic characteristics of an objectbridging one, or comparing the magnetic characteristics of two objectseach bridging a corresponding one of the two air gaps of a stationarymagnetic core shaped to provide two separate magnetic circuits eachincluding a core portion common to both circuits and one of said airgaps, which consists in mosses subjecting the core to magnetizing forcescausing fluxes to flow in the same direction through said common coreportion to, and in parallel through, the two separate circuits and tomagnetizing forces tending to cause a flux to flow in series through thenon-common portions of said separate circuits and for adjusting saidforces until similar magnetic leakages occur across each of said airgaps regardless of the fluxes passing through any magnetic shunt whichmay be applied to either gap.

17 Means for characteristics of a metallic body comprising incombination a stationary magnetic core shaped to provide two separatemagnetic circuits each including a core portion common to both circuitsand an air gap individual to each circuit, amagnetizing windingsurrounding individual portions of the separate magnetic circuitsadjacent each air gap, and means for causing a magnetizing current ofregulated intensity to pass in series through the last mentionedwindings.

18. Means for determining magnetic characteristics of a metallic bodycomprising in combination a stationary magnetic core shaped to providetwo separate magnetic determining magnetic circuits each including acore portion common to both circuits and an air gap individual to eachcircuit, a test coil so disposed with respect to one of said air gaps asto surround a magnetic body bridging said air -vidual to each circuit, atest coil so disposed with respect to one of said air gaps as tosurround a magnetic body bridging said air gap, a second test coilsimilarly disposed with respect to the other gap and similar in thenumber and area of its turns to the first mentioned coil, and ameasuringcoil concentric with and comprising a larger number of turns than saidsecond test coil.

FRANK P. FAHY. Witnesses:

CHAS. W. BURROWS, WM. R. DE LAsHMUTr,

